CAMBRIAN PERIOD (540-500 million years ago)
Cambrian rocks are rich with fossils. Rate of evolution was so quick then, that it is
called the
Cambrian explosion.
Some scientists are of opinion, that during the Cambrian period originated as much as 100
phyla. Our modern 30 phyla may look a bit inconspicuous in comparison with those of the
Cambrian's. First group of organisms - Tommotian fauna, inaugurating the Cambrian
explosion (540-530 million years ago), consists of tiny cones, little coiled shells, round
flattened caps and other. Scientists still don't know if these "small shelly
fossils", as they call them, are little pieces and bits of bigger organisms or main
parts of small animals. Small shelly fauna had spread over the whole Earth, but after 10
million years it began to disappear. It was replaced by new organisms of the subsequent
main pulse of the Cambrian explosion (530 million years ago) which lasted for 5
million years. During this time, almost all of the animal phyla, having hard skeletons,
were established. "The Bryozoa, a group of sessile and colonial marine organisms,
do not arise until the beginning of subsequent, Ordovician period, but this apparent delay
may be an artifact of failure to discover Cambrian representative."
It is hard to say, when exactly animals without skeleton appeared, because they leave
traces only in special circumstances. There are fossils of them from this period, but
almost none from the previous. We don't know the reason why. Maybe, because there were no
such animals or maybe because, yet nobody has found their fossils.
Trilobites
Trilobites were abundant and their fossils are characteristic for this period. There were
about one thousand species of these arthropods. They lived in the sea, crawling on a
bottom, jumping, swimming or digging in the mud. Trilobites lived in shallow warm water as
well as in cool depths. Usually they were 2 to 7 centimeters (1-3 inches) long, but some
of them have even 60 centimeters (2 feet). Trilobites were covered with a hard, calcified
exoskeleton. A typical one had a flat broad head, segmented thorax (2 - 40 segments) with
a pair of limbs on each part.
During the period trilobite species went extinct and
new one appeared. Most of them existed only for a few million years or even shorter.
Toward the end of the Cambrian Period, there happened three great extinctions, one of them
even marks the boundary between the Cambrian and Ordovician Periods. During few thousand
years a total number of trilobites rapidly decreased. Why? Probably because climate
cooled. Trilobites living in warm water were replaced by cool-water species. History of
life was full of this kind of events. Some of them concerned few species, some were global
catastrophes. The biggest happened at the end of Permian Period, on the turn of Paleozoic
and Mesozoic Era.
 
Burgess Shale fauna
One of the most famous Cambrian fossils are these found in shale from the Burgess Pass
in Rocky Mountains, in the south of British Columbia in Canada. These organisms lived in
shallow coastal water on sandbanks that adjoined to calcareous reef built by algae (there
were no reef-building corals then). Most probable is that they were washed away by mud
flows (sort of avalanches) and buried alive in deep water. They preserved thanks to the
low oxygen content.
Fossils from Burgess were found by Charles Doolittle Walcott, an
American geologist and at that time, the chief of the Smithsonian Institution in
Washington, D.C. He discovered them at the end of the 1909 season. Walcott gathered a
collection of 80,000 individuals but having a spate of work, he had no time for
elaborating the importance of his find. He classified Burgess fauna among the younger
fossil and contemporary groups of animals, but they didn't stayed there. Things changed in
the 1960s, when a British paleontologist Harry Whittington began renewed search. He and
his co-workers understood that it is impossible to include Burgess fauna among
contemporary groups. Many of these animals (and plants) have no known successors or
obvious ancestry. Whittington and his collaborators also suggested that the Cambrian
explosion was a chaotic process full of unsuccessful experiments and a game of chance,
rather than an orderly development. First fossils of plant life
appear. In India and Siberia there were found remains of spores, which means that some
lower plants were established. Marine algae abound.
|
ORDOVICIAN PERIOD (500-440 million years ago)
In this period mainly seas and brackish water were inhabited. Maybe some organisms
encroached the rivers. There were many types of mollusks, mainly cephalopods. Some of
them, nautiloids, were 5-7 meters (15-20 feet) long and had a simple 30 centimeters (1
foot) in diameter conical shells. Crinoids and the starfish were represented. New species
of invertebrates, for example reef-building corals, appeared.
In the sandstone from Ordovician Period first vertebrates were found. Fragments of
armored animals - ostracoderms - are preserved in the fresh-water rocks from Canyon City
in Colorado.
Ostracoderms
They were classified among agnathans (jawless fishes). Ostracoderms had a piscine shape
and they were covered with bony plates, scales and shields. Endoskeleton wasn't ossified
or it didn't preserved anywhere. They had not two pairs of fins like true fishes.
Ostracoderms lived on the sea bottom, filtering the water for food. They had no lower jaw
what made biting impossible. They were sucking organisms with a soft body.
|
SILURIAN PERIOD (440-410 million years ago)
Psilophytes
The first land plants, that appeared in fossil form were the psilophytes and phyniphytes.
They reminded later conductive tissue and reproduction by spores. They were several
decimeters (1-2 feet) height and had long, subterranean rhizomes with capillary vessels
instead of roots. Psilophyters had no leaves, but undoubtedly their bare stems were green,
because they needed chlorophyll for nourishment. Sometimes they had scale-like covering.
Perhaps the plants inhabited only marshes and on the dry grounds there
were lichens and liverworts. Evolution of plants by giving feed and shelter facilitated
the development of animals.
The giant sea scorpions made their appearances. They had not the poison-spine and one of
the six pairs of limbs developed into something like a paddle. Some of them attained 2
meters (6 feet) length, and that's probably the reason why ostracoderms had such heavy and
thick armor. They had to protect themselves against those giant arthropods attacks.
Ostracoderms were becoming more numerous. They inhabited lagoons, bays
and inland lakes. Toward the end of this period in the same places first jawed fishes -
placoderms appeared. They were armored with bony plates protecting their head and
front of the body. They also had bony lower jaw and teeth. These big animals (up to 10
meters - about 30 feet - long) were efficient predators.
 
|
DEVONIAN PERIOD (410-360 million years ago)
  Fish became diverse during this period. Placoderms supplanted
ostracoderms, but they had to compete with new groups of fishes: chondrichtyans
(cartilaginous fishes) and bony fishes (Osteichtycs). An extinct class of acanthodians
(spiny sharks) represented by animals with a elongated spindle like body and many fins.
Their remains are found mostly in the sea sediments. Some specialists think that they were
the first fishes to leave fresh water and swim in the oceans and seas. But it is also
possible that the true shark ancestry inhabited terrestrial waters. Bony fishes inhabited
fresh waters. Some of them, lobe-finned or crossopterygian fishes and lung fishes had an
air bladder connected with gullet and it was sometimes used for breathing. Lobe-finned
fishes are believed to be ancestral to the amphibians. The first land vertebrates had many
structural features that indicate a close kinship with the lobe-finned fish. It was long
thought that the crossopterygians fishes had been extinct for the last 70 million years,
but living represenatives called coelacanths have been caught off the coast of South
Africa. The appearance of amphibians may have been not the effect of evolution of only one
group of fishes. There were many parallel evolution lines from among some survived. The
other air-breathing animals in this period were invertebrates, for example snails,
spiders, mites. Toward the end of this period millipedes and wingless insects appear.
Photos of Coelaconths courtesy of Museum of Evolution in Warsaw
|
CARBONIFEROUS PERIOD (360-290 million years ago)
   It is sometimes divided
into Mississipian and Pennsylvanian, especially in North America. Lands were covered with
swampy forests in which grew giant horsetails, lycopods, ferns, seed ferns and gymnosperms
forming the coal beds. Most of the coal deposits originate from this period. Large marine
sharks were abound. The sea lilies (croniods) became very numerous and reached their peak
in this period. The amphibian continued to increase. First bryophytes (mosses) appeared.
The amphibians called stegocephalians because of their armored head, dominate the picture.
The first reptile showing its ancestry in the retention of many stegocephalians
characteristics occurred in this period. Compared with our modern insects the
Carboniferous insects were of tremendous size. A fossil dragonfly recovered from this
period had a wingspread of 76 centimeters (30 inches) and a length of 38 centimeters
(15 inches).
Photos of dragonfly, seed ferns and tree lycopods (club-mosses) bark
imprints courtesy of Museum of Evolution in Warsaw
|
PERMIAN PERIOD (290-250 million years ago)
This period witnessed the decline of the lycopods (clubmosses)
and horsetails and the higher development of the gymnosperms. Cycads and conifers appeared
for the first time. The trilobites and ostracoderms became extinct. Smaller insects
capable of complete metamorphosies are found in this period. The primitive reptiles
increased, most of them being lizard like in form. Some of primitive reptiles, called
Theriodontia, possessed certain mammal characteristics, one of which is exemplified in the
differentiation of their teeth into incisors, canines and molars. Paleontologists believe
them to be the "missing link" between the reptiles and mammals. The ancient
amphibians declined. During this period swamps and inland seas were drained, as the
continents formed a great supercontinent of Pangea.
"The mother of mass extinctions"
People were always fascinated by dinosaurs, so the best known extinction
is that from between Mesozoic and Cenozoic epochs 65 million years ago when dinosaurs
disappeared. However the greatest disaster happened at the end of Permian Period, some 250
million years ago. "Affectionately called "the mother of mass extinctions"
among paleontologists, it yielded a death toll that is truly staggering." Almost half
of all families, about 70 percent of genera disappeared. The most attended organisms were
marine individuals, especially corals, articulate brachiopods, bryzoans, echinoderms (sea
lilies), trilobites, shallow-water foraminifera, ammonoids. Before this event oceans were
inhabited mainly by immobile organisms, attached to the seafloor, filtering organic
material from water. As a result of the catastrophe 90 percent of all marine species
vanished. Without much losses escaped snails, bivalves, nautiloids. Only conodonts
resisted the extinction. Their mouthparts are abundant and even serve as markers of time.
Disappearance of one groups was a chance to expand for another. After
the disaster, active predatory individuals became much more numerous.
On land 78 percent of reptile and 67 percent of amphibian families
became extinct. Among insects, from 27 Permian orders 8 disappeared, 3 hardly survived
till Triassic, but soon became extinct too, and 4 suffered great losses. It is the only
known extinction in their history. The flora was also attended, but having insufficient
evidences, it is impossible to say on what scale.
What was the reason? Probably there were several causes, massive
volcanic eruptions and changes in sea level, which interacted.
The History of Life The History of Universe
Links:
http://www.paleo.pan.pl/museum/museum.htm
|
